The Foreign Secretary raised the prospect of UK forces getting involved if there were more chemical weapons attacks in the troubled country.
The government gave strong backing to Donald Trump’s retaliation against Assad earlier this month, after an apparent sarin atrocity killed dozens of civilians.
But British forces were not involved on that occasion, when missiles destroyed a regime airbase.
The incident caused a major diplomatic standoff with Russia, which has been staunchly supporting Assad.
Ministers refused to be drawn on whether they were prepared to get involved in future action.
Asked in a round of broadcast interviews today whether the UK would join action if the US requested it, Mr Johnson replied that ‘it will be very difficult to say no’.
He added that was also the view of Prime Minister Theresa May.
Mr Johnson told BBC Radio 4’s Today programme: ‘I think it will be very difficult if the United States has a proposal to have some sort of action in response to a chemical weapons attack, and if they come to us and ask for our support, whether it’s with submarine-based cruise missiles in the (Mediterranean), or whatever it happens to be…
‘It would be in my view, and I know this is also the view of the Prime Minister, it would be very difficult for us to say no.’
Asked if the PM would have to win a Commons vote before taking action, he replied: ‘I think that needs to be tested.
‘I think it would be very difficult for us to say no.’
Asked if the approval of MPs was a necessary pre-condition, he replied: ‘As I said, I think it would be very difficult for us to say no; how exactly we were able to implement that would be for the Government, for the Prime Minister.
‘But if the Americans were once again to be forced by the actions of the Assad regime – don’t forget, it was Assad who unleashed murder upon his own citizens with weapons that were banned almost 100 years ago – if the Americans choose to act again and they ask us to help, as I say, I think it would be very difficult to say no.’
The comments came as the Foreign Secretary ripped apart Jeremy Corbyn’s record on security issues.
Mr Johnson highlighted his lukewarm support for Nato, his opposition to Trident, and his refusal to countenance ever using Britain’s nuclear deterrent.
And he warned that one of the main dangers of the election was that people did not take the prospect of Corbyn in No10 seriously.
‘They watch his meandering and nonsensical questions and they feel a terrible twinge of human compassion. Well, they say to themselves: ‘he may be a mutton-headed old mugwump, but he is probably harmless’,’ Mr Johnson wrote in The Sun.
‘Have you felt a pang of sympathy for his plight? If so, fight it. The biggest risk with Jeremy Corbyn is that people just don’t get what a threat he really is’.